For a general entangling probe attacking the BB84 protocol in quantum key distrobution, I show that the simplest quantum circuit representing the optimal entangling probe, and yielding the maximum information gain on the pre-privacy-amplified key, consisting of a single CNOT gate in which the control qubit consists of two polarization-basis states of the signal, the target qubit consists of two probe-photon polarization basis states, and the initial state of the probe is set by the error rate induced by the probe. A method is determined for measuring the appropriate correlated state of the probe. Finally, a possible implementation of the entangling probe is described. the device is a simple special-purpose quantum cryptography.